A normal telescope optical system comprises, in order from an object, an objective lens system and an eyepiece lens system. An Image of a distant object is formed by the objective lens system on the focal plane thereof, and the formed image is magnified by the eyepiece lens system and viewed.
The objective lens system generally includes a cemented lens comprising cementing of a lens having positive refractive power and a lens having negative refractive power. The cemented lens generates and cancels out positive and negative aberrations, thereby correcting axial chromatic aberration. The eyepiece lens system generally comprises two or more lenses, and corrects lateral chromatic aberration through setting an appropriate spacing between the lenses. Alternatively, in an instance in which the eyepiece lens system includes a cemented lens comprising cementing of a lens having negative refractive power and a lens having positive refractive power, the lateral chromatic aberration is balanced using the cemented lens or by setting an appropriate lens spacing between the cemented lens and other lenses.
Meanwhile, in recent years, diffractive optical elements having a configuration that is drastically different from conventional optical lenses are used in a variety of optical systems in order to, e.g., reduce various aberrations. There have been proposed objective lens systems in which a diffractive optical element is used in order to reduce the occurrence of various aberrations, particularly axial chromatic aberration (e.g., see Patent Document 1). There have also been proposed eyepiece lens systems in which a diffractive optical element is used in order to reduce the occurrence of various aberrations, particularly lateral chromatic aberration (e.g., see Patent Document 2).